1. Field of the Invention
The present invention relates to a solid electrolytic capacitor, the solid electrolytic capacitor comprising an anode of valve metals or alloy of which main component is such valve metals, a dielectric layer formed by anodizing said anode, and a cathode formed on said dielectric layer. More particularly, a feature of the invention is to prevent increase of leakage current in heat treatment process, such as reflow soldering process, and to increase capacitance.
2. Description of the Related Art
Generally, it has been proposed to fabricate a solid electrolytic capacitor by: anodizing an anode of valve metals, such as titanium, niobium, tantalum and the like or of alloy of which main component is such valve metals, in phosphate solution to form a dielectric layer of oxide on the anode surface; overlaying an electrolyte layer composed of an oxide having a conductivity or a conductive polymer on the dielectric layer; and overlaying a carbon layer and a silver paint layer as a cathode on the electrolyte layer. (See, for example, Japanese Published Unexamined Patent Application No.6-151258 and Japanese Published Unexamined Patent Application No. 2004-18966.)
However, the dielectric layer of oxide in such a solid electrolytic capacitor is easily affected by heat. Especially, the dielectric layer formed by anodizing an anode using niobium or titanium is easy to be greatly affected by heat, and there has been a problem when heat treatment process, such as reflow soldering process, is conducted, a crack in the dielectric layer occurs, for example, by expansion and contraction of the electrolyte layer, or increase of leakage current is caused, for example, by crystallization of the dielectric layer.
In recent years, in order to provide a solid electrolytic capacitor having little changes in capacitance by heating of a reflow soldering process, a solid electrolytic capacitor in which a dielectric layer composed of a niobium oxide layer and a niobium nitride region is formed on the surface of an anode of niobium has been proposed. (See, for example, Japanese Published Unexamined Patent Application No. 11-329902.)
Nevertheless, a problem in the above-described solid electrolytic capacitor in which the dielectric layer composed of the niobium oxide layer and the niobium nitride region is formed on the surface of the anode of niobium has been that it is still impossible, in the case of reflow soldering process, to fully suppress occurrence of the crack in the dielectric layer or of crystallization of the dielectric layer which result in increase of leakage current.